The present invention relates generally to a fixation device for fixing fractures. In particular, the present invention relates to an orthopedic fixation device adopted to fix intra-articular hand fractures or fracture subluxation or fracture dislocation in finger joints internally or externally.
In hand surgery, difficult fractures and fracture subluxation or dislocation are frequently treated by open reduction and internal fixation. The proximal interphalangeal joint of fingers is an especially difficult region for treating fractures.
As the fracture is intra-articular, anatomical reduction is crucial to regain joint function and prevent early degenerative changes of the joint. The fracture fixation device must be able to resist the physiological deforming forces namely axial compression, bending and torsion. These physiological deforming forces exert high constant loading to the fracture as the finger flexor muscles and finger extensor muscles have a strong resting muscle tones. These muscles act like rubber bands pulling on the distal end of the digit and constantly acting as a deforming force to displace the fracture.
Moreover, when comminuted fractures occur, articular fragments may collapse and cause joint incongruity. Joint incongruity is undesirable as it will cause early cartilage damage and hence degenerative joint changes.
Juxta-articular fracture is another difficult fracture type. It refers to fracture at the head and the base of the proximal phalanx and middle phalanx.
Head fractures or basal fracture are problematic as the fragments are small and intra-articular. The fragments are linked by the ligaments of the joint. They are difficult to fix by an internal fixation device due to the small size and difficulty in surgical approach. However, to achieve a good functional outcome, the margin of error in reduction is within 1 mm. The flexor and extensor muscles again are a constant deforming force on the internal fixation device to displace the fracture after internal fixation.
Conventionally, inter-phalangeal joint dislocation/subluxation and juxta-articular fractures are treated with open reduction and internal fixation. Fixation devices include small screws or a small metallic pin.
For tiny fracture fragments which cannot be fixed, an external fixator can be used to provide tractional force to the involved joint. The fracture fragments can be reduced by this indirect method which is called ligamentotaxis.
In the worst scenario of comminuted fracture involving the proxmial interphalangeal joint, the joint may need to be fused as fixation is not possible. A fusion will sacrifice all the movement to allow the joint to heal in a useful position, usually at 30-50% flexion.
Other alternatives are artificial proximal interphalangeal joint replacement or joint transfer from another part of the body, usually a toe joint. However, artificial joints are not long lasting and the use of the joint requires the sacrifice of a toe joint.
It is desirable to provide a fixation device to address the above problems. The present invention provides such a fixation device that can reduce these fractures, maintain the fixation by counteracting the physiological deforming force, and hence can replace the conventional treatment methods.
This fixation device can be comprised of a temperature sensitive material such as Nitinol. Nitinol (NiTi) belongs to the family of shape memory alloys, which have been discovered in recent decades. These shape memory alloys have 2 states in solid, one is martensite state and the other is an austenite state. When the material is heated or is above its transformation temperature, it will return to its predetermined shape and it is in an austenite state. In the austenite state, the material is very strong. When the material is cooled, or is below its transformation temperature, it will become soft and pliable. At this temperature, it is in a martensite state and it can easily be manipulated into other shapes.
Another property of shape memory alloys is its supra-elasticity. In the austenite state, the material is very strong, when it is exposed to a critical load, stress will induce martensite transformation. Below this critical loading, the internal stress of Nitinol remains constant despite an increase in strain. By using this property, devices made from Nitinol can generate a constant, continuous force. The force can be compression, distraction, bending or torsion.
The transformation temperature of Nitinol can be determined during the production of the implant. The shapes in the martensite state and in austenite can also be predetermined during the manufacturing procedure. For the purpose of a fixation device in the human body, the transformation temperature can be made at around 25-37° C., preferably 25-35° C. At this temperature, the Nitinol device would be hard and strong. When the device is cooled below the transformation temperature e.g. at 20° C., the device is soft and pliable, i.e. it is in its martensite state. In this soft martensite state, manipulation is easy and placement of a device to a fracture site will be easy and comfortable. There is no need to re-bend the device such as like using a stainless steel implant. The fixation device is expected to be very user-friendly, cooling a Nitinol device to 20° C. is easy as one can just submerge it in cold water. The device can be heated to above the transformation temperature with a simple electrical heating system. When the fixation device is in the body, the body temperature will keep it in the austenite state. The device will continue to be strong and is able to resist high forces.
Because of the supra-elasticity effect of Nitinol, it can generate a constant force when it is in use. When rigid internal fixation is adopted to treat a fracture, fixation in compression is essential to eliminate all the micromotion at a fracture site. Conventional fixation methods include a lag screw or a plate in compression. The compression force is determined at the time of operation. If there is subsequent bone resorption around the screw holes, the compression force will be lost. The target of primary bone healing will not achieved. If the compression force is generated by the supra-elastic property of Nitinol, this compression force is constant as long as the Nitinol device is in situ. What occurs is more resorption around the implant insertion site, the force will still be constant.
A Distraction force is required for treatment of intra-articular fracture, fracture subluxation/dislocation, bone lengthening or soft tissue lengthening. The traditional method of using a distraction device use includes using an external fixator connected to a threaded rod or traction with spring or rubber band. As the tissue lengthens, the tension will decrease. However, the distraction force generated by conventional method is not constant. It is desirable for a distraction device to produce an adequate and constant distraction force. The supra-elastic property of Nitinol can serve this purpose.